1. Field of the Invention
The invention relates to a diesel fuel composition for use in internal combustion engines.
2. Brief Description of Related Technology
Conventional diesel fuels are used throughout the United States and the world in internal combustion engines to power a wide variety of vehicles such as, for example, farm equipment, passenger cars, buses, trucks, and construction equipment. There are generally two conventional diesel fuels used in these types of vehicles, i.e., No. 1 diesel fuel and No. 2 diesel fuel. Conventional diesel fuels, however, are disfavored by both consumers and regulators because of the loud engine noise and harmful emissions (smoke) caused by the combustion of the fuel, and the hard starting in cold weather conditions. Accordingly, gasoline has gained widespread use and today dominates the market for fuels used in combustion engines. Diesel combustion engines, however, provide significant advantages over engines using gasoline fuels, including improved energy efficiencies. Diesel fuels contain higher energy then gasoline with a typical gallon of No. 2 diesel containing over 140,000 Btus compared to 115,000 Btus in a gallon of gasoline.
Hybrid diesel fuel formulations have been developed to address various problems associated with diesel fuels and their combustion. For example, for purposes of economics, combustion characteristics, and availability, ethanol has been used in hybrid diesel fuel formulations. While anhydrous ethanol and diesel fuel are miscible at room temperature, trace amounts of water in the mixture may cause phase separation when the ethanol is mixed with the diesel fuel. Furthermore, as the temperature is lowered, the fuel's tolerance to take on water is diminished. Water present in the diesel fuel undesirably separates from the fuel to form an immiscible layer. This water aggregate is undesirable as it leads to erratic combustion, poor combustion emissions, and could corrode components of the fuel delivery system, storage, and combustion engine.
To address water contamination problem, conventional diesel fuel industry typically adds de-emulsyfying agents that repel water into a separate layer during storage. However, this provides little or no protection against water exposure during fuel use. Presence of separated water in a compression ignition diesel engine results in a sudden loss of power and erratic drivability. Hence techniques based on integrating the water into diesel fuel rather then repelling it from the fuel promise better performance and combustion characteristics.
Hybrid diesel fuel emulsions and microemulsions have been developed to improve the water tolerance of diesel fuels. Such emulsions and microemulsions include, for example, a mixture of the diesel fuel, water, an alcohol, and a combination of surface-active agents made up of a variety of salts of long-chain fatty acids. See e.g., U.S. Pat. No. 4,083,698. U.S. Pat. No. 4,451,265 describes a microemulsion containing diesel fuel, water, water-miscible alcohols, and a surfactant system using N,N-dimethyl ethanolamine and a long-chain fatty acid substance. A major disadvantage of emulsion and microemulsion fuel formulations, however, is the lack of stability (i.e., temperature and time stability) under the type of conditions which the fuel formulations can be expected to encounter. In general, microemulsions have a tendency to de-emulsify under increased pressures, such as those experienced in compression-ignition diesel engines. Microemulsions also have a tendency to de-emulsify at high and low temperatures. Efforts to stabilize microemulsions over a temperature range of -20.degree. C. to +70.degree. C. are taught by U.S. Pat. No. 4,744,796. Regardless of these advances, however, emulsions and microemulsions have physical properties which limit their use in unmodified combustion engines.
Additionally, hybrid diesel fuel formulations require time-intensive and energy-intensive preparation procedure(s). Because it is very difficult to homogenize a mixture of a low molecular weight alcohol, such as ethanol, and higher molecular weight hydrocarbons, such as diesel fuel, most emulsion and microemulsion fuels require expensive blending operations employing an emulsifying agent. For example, such emulsions are typically prepared by vigorous mixing, recirculating, and heating (e.g., to a temperature of about 50.degree. C. or more, for example about 72.degree. C.) for about 20 minutes to provide a homogenized emulsion, usually opaque in appearance. This energy intensive process translates into significant economic penalty resulting into a product with little or no commercial viability.
Accordingly, it would be desirable to provide a temperature-and time-stable, clear diesel fuel composition for use in combustion engines, and preferably unmodified combustion engines, that more closely emulates the physical properties of conventional diesel fuel, yet employs less of the actual base diesel fuel. Additionally, it would be desirable to provide a diesel fuel formulation that can accommodate water contamination. Furthermore, it would be desirable to provide a fuel composition that has improved emissions as compared to a base diesel fuel (either No. 1 diesel fuel or No. 2 diesel fuel). Still further, it would be desirable to provide a fuel composition that can be prepared readily without the need for energy-intensive blending procedures.